Ceramic honeycomb structural bodies are effective as catalyst carriers for purifying exhaust gases of internal combustion engines and as filters for fine particle removal. These honeycomb structural bodies are usually made of a ceramic material such as cordierite, alumina, silicon carbide, and the like, and owing to their configurations, they are generally produced by an extrusion method.
In order for honeycomb structures to function efficiently as catalytic converters, it is necessary that their cells provide a substantially large surface area for catalytic material to react with the exhaust gases. Also, the cell walls must have a substantially thin cross-sectional dimension to provide a large open frontal area to reduce back pressure within the exhaust system. However, the thin walled structure must have sufficient mechanical and thermal integrity to withstand normal automotive impact and thermal requirements.
In the past, it has been customary to form extrusion dies for forming thin-walled honeycomb structures from a solid die body by saw-cutting discharge passages in the outlet face of the die body and drilling rather lengthy feed holes into the inlet face of the die body which communicate with such discharge passages, as shown in U.S. Pat. No. 3,790,654 to Bagley. As further shown in the Bagley patent, the feed holes may communicate with each intersecting passage or every other intersection, as desired. However, in both cases the feed holes extend a substantial distance through a unitary die body.
In order for the extruded material to coalesce within the discharge passages, it is necessary for the passages to have sufficient length so that the extruded material will have time to flow transversely within slots to knit into a unitary grid prior to being longitudinally discharged from the outlet face of the die. Alternatively, additional feed holes may be provided in communication with the slot gridwork to reduce the amount of transverse flow required to provide such a unitary cellular matrix prior to discharge from the die face.
In the past, planar extrusion dies have been utilized to form ceramic honeycomb structures. Examples of such extrusion systems are disclosed in U.S. Pat. Nos. 3,836,302 to Kaukeinen, 4,118,456 to Blanding, et al., 4,687,433 to Ozachi, et al., and 4,877,766 to Frost. Such devices work well in extruding single piece structures having relatively small cross-sectional areas. The cross-sectional area of honeycomb structures is, however, limited by the size of the die used to form them. This becomes a problem when it is necessary to produce honeycomb structures with large cross-sectional areas. It is simply not cost-effective to manufacture a large one-piece honeycomb structure with a planar extrusion die. Instead, small component pieces of such large structures are separately extruded through planar dies, and the pieces are then fitted together and sealed with frit. Due to its time-consuming and labor-intensive nature, this procedure is not particularly satisfactory. There, thus, continues to be a need for techniques of producing large ceramic honeycomb materials in an efficient and cost-effective fashion.